The present invention is related in a general way to processes for the treatment of ore to prepare it as raw material for steel-making processes, notably those involving steel-making furnaces, and is particularly related to the process of preparing sinter.
Presently sinter represents the main raw material used in the world for feeding steel-making furnaces. Sinter is formed when (1) a certain portion of iron ore and/or manganese is mixed in defined proportions with coke or coal and other additives, (2) the mixture is subjected to high temperatures derived from the combustion of this coke or coal, and (3) it is agglomerated by incipient fusion of the particles of ore so as to form solid sinter particles having considerable porosity and mechanical resistance. These sinter particles are ideal for the feeding of steel-making furnaces.
The size range of the ore used in the sintering process is, however, limited in its lower range. In particular ore having more than 40% of its particles with a size below 150 microns is unacceptable for this purpose because of the risk of attaining an economically unacceptable yield in the process of sintering. The low yield is due to the obstruction by the fine ore particles of the passage of the gases necessary for the combustion of the coke or coal. Therefore, a great number of iron ore and/or manganese mining companies are faced with the problem of disposing of considerable stocks of ore fines (i.e. ore with a substantial proportion of particle sizes below 150 microns) which fines cannot be used for sintering. Part of these fines, however, can be pelletized, after which they are used directly in the steel-making furnaces without being sintered.
Pelletizing consists in the formation of spheres of ores of a diameter from 6 to 18 mm by discs, cones or pelletizing drums and the hardening of these spheres in special furnaces at temperatures averaging 1300.degree.-1400.degree. C. This process, however, is costly since it involves a high consumption of combustibles and power.
Other pelletizing processes of restricted use are also known. In these other processes, pellets of conventional size (6 to 18 mm) are produced in discs, cones or pelletizer drums from ore to which cement has been added, and the pellets are hardened by heat treatment at lower temperatures. These pellets are also designed for the direct feeding of blast furnaces.
There is, however, the need for a process which allows for the use of fines with particle size ranges below 150 microns at a cost below that of the conventional pelletizing processes.